1. Field of the Invention
The present invention relates to operation, administration, and maintenance (hereinafter, referred to as “OAM”) functions in Ethernet passive optical network.
2. Description of the Related Art
Currently, the standardization of the Media Access control (MAC) technologies for a Gigabit Ethernet and an Asynchronous Transfer Mode Passive Optical Network (hereinafter, referred to as “ATM-PON”) have been completed, the details of which are described in the IEEE 802.3z and in the International Telecommunication Union-Telecommunication standardization sector (ITU-T) G983.1.
In an ATM-PON having a tree-shaped structure, ATM cells are transmitted upward and downward in the units of frames having a predetermined size. An optical line termination (OLT) is provided to transmit downward cells, which will be distributed to each optical network unit (ONU).
FIG. 1 illustrates a physical network architecture of a general passive optical network. As shown, a passive optical network includes an OLT 100 and a plurality of ONU 110-1 to 110-3 connected to the OLT 100. In particular, FIG. 1 shows an example in which an OLT 100 and three ONUs 110-1 to 110-3 are connected with each other, and each of the ONUs 110-1 to 110-3 is connected to at least an end user (a user device or a network device) 120-1 to 120-3. In operation, data 131 to 133 outputted from each end user 120-1 to 120-3 are transmitted to the OLT 100 through a corresponding ONU 110-1 to 110-3.
In the optical network (hereinafter, referred to as “EPON”) shown in FIG. 1, an IEEE 802.3 Ethernet frame is transmitted through a point-to-multipoint network according to a time division multiplexing (TDM) method. To avoid data collision, a method called ‘ranging’ is implemented in an optical distribution network (ODN), which is a passive device. That is, during an upward transmission, the data from each of the ONUs 110-1 to 110-3 is transmitted to the OLT 100 in a multiplexed state, and downward transmission is performed in such a manner that the ONUs 110-1 to 110-3 selectively receives the intended data from the OLT 100. To this end, each of upward and downward frames has a dedicated ATM cell or a field allocated in a normal ATM cell so as to transmit and receive messages at regular intervals.
With the development of Internet technology, the demand for more bandwidth is growing steadily. To address the need, the development for end-to-end transmission in the Gigabit Ethernet, which is relatively low-priced and can secure a high bandwidth, has been explored over the ATM technology. The ATM has drawbacks in that it is relatively high-priced, has a limited bandwidth, and must perform a segmentation process for each IP packet. Thus, the PON architecture favors to be operated in the Ethernet mode instead of the ATM technology.
The EPON standard issued by the IEEE 802.3ah is in progress under the name of “Ethernet in the First Mile (EFM)” and targeted for September 2003. Draft v1.2 is currently in progress, and Draft v2.0 is expected to be completed on November 2003. FIG. 2 illustrates the format of an OAM packet 200 proposed in the Draft v1.2.
In order to perform a loop-back test between a local device and a remote device, the local device transmits a loop-back control OAM PDU (Packet Data Unit) to the remote device. Note that the loop-back control OAM PDU includes a loop-back time determined by the local device. The remote device, having received the loop-back control OAM PDU, is changed into the loop-back state and transmits the loop-back state information of the remote device to the local device using an OAM PDU information, thereby initiating the loop-back process.
After the loop-back test is performed for a predetermined time, a termination of the loop-back test is performed. There are two methods to perform the termination process. In the first method, the local device tries to terminate a loop-back test. In a second method, the remote device tries to terminate a loop-back test. According to the first method, when the local device tries to terminate a loop-back test, the local device transmits a loop-back control OAM PDU to the remote device, during which the loop-back control OAM PDU includes a loop-back time determined as ‘0’. Then, the remote device receives the loop-back control OAM PDU including a loop-back time of ‘0’, and sets the loop-back time to ‘0’ in order to terminate the loop-back test. Subsequently, the remote device transmits an OAM PDU information containing a changed loop-back state to the local device. The local device receives the OAM PDU information transmitted from the remote device and then transmits its own changed loop-back state—a loop-back stop state—to the remote device, so that the loop-back process can be terminated.
The second method of the termination process involves the termination of a loop-back time, which is determined by the local device with the start of the loop-back process. That is, as the loop-back time determined by the local device is terminated in the remote device, the remote device transmits an OAM PDU information, in which a changed loop-back state is included, to the local device. Then, the local device, in response to this, transmits its own loop-back state—a loop-back stop state—to the remote device, so that the loop-back process is terminated.
As described above, in order to perform the start and the termination of a loop-back, a local device and a remote device exchange two kinds of messages—a loop-back control OAM PDU and an OAM PDU information. Accordingly, the prior art has disadvantages in that the complexity of the systems are increased due to the inefficient message exchange protocol.